METHODS: The study will be done on the General Clinical Research Center at Duke University Medical Center, on an inpatient basis (depending on where the patient lives). No one who is suspected of being infectious or is infectious from TB will be enrolled on the GCRC. After informed consent is obtained, each subject will be admitted to the GCRC twice; the first admission will occur after at least four twice weekly doses of intermittent rifabutin and prior to beginning antiretroviral therapy and the second admission will occur two to six weeks following the institution of an antiretroviral regimen including efavirenz. During Admission #1, blood will be drawn at 0, 1, 3, 5, 7, 9, and 21 hours post dosing with INH and rifabutin. During Admission #2, blood will be drawn at 0, 2, 4, 6, 8, 10, 12, and 24 hours. Efavirenz will be given at time points 0 and 24 hours post sampling. TB drugs will be given at 3 hours (so that sampling times subsequent to INH/rifabutin dosing will correspond to those of Admission #1). Patients will be interviewed regarding concomitant medications, gastrointestinal symptoms and meals relative to study drug dosing. Sixty days following the last dose of PK study medicines, a follow-up visit or phone call (including review of medical record) will identify any adverse events.